I think Singapore math is excellent. No need for the workbook if he is doing the IP.
My son math minded too. We started with SM 1 and 2, then went to Beast Academy 3. 1 and 2 were not out back then. Then straight to Art of Problem Solving pre-algebra.
I will say that I did do BA 2A and 2B with one of my daughters and was not a big fan. But enjoyed BA 3, 4, and 5.
Beast Academy is a lot harder than Singapore Math, so something to consider. Otherwise, I think SM is an excellent program.
Hope this helps.